Ontogenetic variation in light requirements of juvenile rainforest evergreens

1. Although shade tolerance is often assumed to be a fixed trait of species, recent work has reported size-related changes in the relative and absolute light requirements of woody taxa. We hypothesized that, in evergreen forests, light requirements of shade-tolerant species that accumulate multiple foliage cohorts will be more stable during juvenile ontogeny than those of intolerant species with short leaf lifetimes. 2. We quantified the light environments occupied by three size classes of 13 coexisting evergreens in a temperate rainforest, to determine how size influenced their relative shade tolerance. Minimum light requirements (MLRs) of species were estimated by computing the 10th percentile of the distribution of juveniles in relation to percentage canopy openness, for each size class. Leaf life span in low light (2%-5% canopy openness) was estimated by recording survival of marked leaves over 12 months, or retrospectively on species with clearly discernible foliage cohorts. 3. Agreement of ranks of species' MLR across size classes was significant, although not strong (Kendall's W = 0.159, P = 0.02). MLRs of the most shade-tolerant species changed little between size-classes, whereas those of most of the less-tolerant species rose with increasing size. 4. Shift in MLR across size-classes was negatively correlated with leaf life span, possibly because of the effects of leaf life span on biomass distribution and whole-plant carbon balance. Survival of light-demanding species with short leaf lifetimes may thus depend on their encountering increasing light levels as they grow taller, whereas progressive accumulation of an extensive leaf area by late-successional taxa enables them to continue to tolerate low light despite increasing size. 5. Results suggest that shade-tolerance differences between evergreens become increasingly apparent with increasing size. In identifying a relationship with leaf life span, this work also provides a basis for predicting changes in species' light requirements during juvenile ontogeny.